The present invention relates to an equipment for the injection molding of bars for spectacles, made of plastic material and incorporating a metal stem or core onto which the respective metal hinge is previously welded, or obtained in one piece therewith.
As everyone knows, it is very difficult to produce--in a single injection molding stage--finished bars for spectacles, already incorporating a stiffening metal stem or core.
In actual fact, if one prepares a mold in which the metal stem is arranged in a central position and held by its end provided with hinge, at the moment of injecting the plastic material--which should surround and envelop the core--the speed and pressure at which said material enters the mold, cause the curling up of the stem.
All the attempts made to hold the stem more firmly into the mold, during the injection of the plastic material, have always given negative results.
Different techniques--more or less complicated and anyhow comprising several working stages--have therefore been adopted for producing spectacle bars of the aforementioned type.
According to a first technique, one produces first of all a bar preform by injection molding. In a subsequent working stage, the preform is heated and a metal core, provided at one end with the respective hinge, is introduced with its other end into the preform, wherein it is caused to slide along the longitudinal axis thereof. The preform is heated to the softening point, so as to allow the introduction therein of the pointed and relatively narrow metal core. Obviously, at the end of said operation of introduction, the preform undergoes further treatment so as to acquire the final shape of the spectacle bar.
As can easily be realized, this technique involves:
two working stages,
the need to work on one bar at a time,
a high percentage of defective pieces,
a limitation in the shapes and sizes of the finished bars.
According to a more recent technique, it has been proposed to introduce the metal cores in the actual molding machine, immediately after the injection stage. To obtain this result, one uses a mold which is closed, at the end of the bar which will carry the hinge, by a knife element, a guide for the metal core being provided beyond said knife element and along the extension of the longitudinal axis of the bar. At the end of the injection stage and as soon as the mold has been filled with molten plastic material, after having interrupted the feed pressure, the closing knife is opened and the metal core is caused to slide axially into its guide by being thrust at one end and being introduced with the other end into the mold.
To make sure that the metal core--which, as already said, is very thin and flexible--does not bend during this operation of introduction (which tends to be opposed by the plastic material present in the mold, though at the semimolten state), it is indispensable for the core to fit exactly in its guide. It is thus excluded that also the hinge of the bar may slide into the same guide.
To adopt this technique, the metal cores are therefore perfectly rectilinear, with no hinges or other projecting parts. At the end of the molding stage and after introduction of the metal core, the bar will be provided with its hinge in a subsequent working stage. The hinge is usually fixed by riveting or similar techniques.
Though this system has certain advantages compared to that previously described, in that:
in a single working stage one obtains the bar in its final shape and with the metal core inserted therein, and
in a single subsequent working stage the hinge is applied; nevertheless, it is not always the preferred system, on account of the fact that:
the technique of applying the hinge by riveting is delicate,
the fixing obtained thereby is not so firm and lasting as in the case of hinges welded on the core,
the global aesthetical effect is not always acceptable, since a hinge thus mounted is quite massive and bulky.